Due to the recent development for high density and high precision of highly integrated circuits such as super-LSI devices, the requirements for smoothness and low-defect quality of a substrate for electronic devices such as a mask blank substrate are becoming stricter year by year.
Some known examples of conventional methods for reducing surface roughness of a principal surface of a glass substrate for a mask blank to enhance smoothness and low-defect quality include a method in which a principal surface of a synthetic quartz glass substrate is cleaned with low concentration hydrofluoric acid solution after precision polishing of the principal surface using a polishing liquid containing polishing grain including oxides such as colloidal silica, or before forming a film on the principal surface (Patent Document 1), a method of processing with a low concentration hydrofluoric acid solution followed by cleaning with alkaline liquid (Patent Document 2), and a method in which a surface of a glass substrate is polished using a double side polishing device with polishing slurry containing colloidal silica, followed by cleaning with sulfuric acid/hydrogen peroxide/water mixture (Patent Document 3).
Recently, further shortening of wavelength of exposure wavelength is in progress to cope with the requirement for more minute patterning of highly integrated circuits such as super-LSI devices as mentioned above. Particularly, development is in progress for reflective mask blanks for EUV exposure using Extreme Ultra Violet (hereafter referred to as EUV) light with a wavelength of about 0.2 to 100 nm as an exposure light.
However, in cases where, for example, ArF laser light (wavelength 193 nm) is used as an exposure light, there is a disadvantage that even a negligible defect, e.g., a minute defect around 30 nm, may cause a problem in a mask blank for EUV exposure using EUV light of, e.g., exposure wavelength of 13.5 nm with a minute defect greater than the exposure wavelength.
Based on the above, a substrate process is being developed so as to prevent a defect of such a critical size from remaining on the principal surface of the mask blank substrate.
On the other hand, development for high sensitivity is in progress on defect inspection devices for inspecting microscopic defects of the above size. Particularly, when the defect inspection criterion is enlarged to more minute defects in a defect inspection device for bright-field illumination, a false defect problem may occur which detects a background noise caused by a surface roughness of the substrate principal surface as a defect.
Therefore, smoothness of 0.08 nm or below at Rms (root mean square surface roughness) is demanded for a mask blank substrate for EUV exposure.
Recently, Catalyst Referred Etching (also referred to as CARE) method has been suggested (Patent Document 4).
The CARE method is conducted by causing a principal surface of a crystalline substrate such as SiC and a catalyst to be close to, or in contact with each other with a processing liquid such as an acidic liquid therebetween, and using active species produced from molecules in the processing liquid adsorbed to the catalyst, selectively removing, by mechanical processing and polishing, microscopic protrusions (machining-affected layer) generated as a crystal defect on the principal surface, to thereby achieve flattening and smoothening of the principal surface.